1. Field of the Invention
This invention relates to a novel powder coating composition based on copolymers having epoxide groups and cyclic carboxylic anhydride groups and acids or acid derivatives as the hardener and to the use of this powder coating composition for coating heat-resistant substrates, in particular for the production of automotive clear lacquers.
2. Description of the Prior Art
Powder coating compositions based on epoxy-/functional copolymers which contain carboxylic acids, in particular dicarboxylic acids or dicarboxylic anhydrides as the hardener are known and described in DE-AS 2,240,312, DE-AS 2,240,314, DE-OS 2,457,826 and U.S. Pat. No. 4,091,048.
Copolymers containing functional groups in addition to epoxide groups, for example hydroxyl, anhydride or amide groups, may also be used in the production of powder coating compositions (c.f. for example, DE-OS 2,441,624, DE-OS 2,441,752, DE-OS 2,441,753, DE-OS 2,457,827, DE-OS 2,457,894, DE-OS 2,509,410, U.S. Pat. No. 3,932,367, U.S. Pat. No. 3,991,132 and U.S. Pat. No. 4,374,954).
The powder coating compositions described in these publications were developed in particular for automotive clear coats. However, the resulting coatings do not completely fulfil the current property requirements with regard to resistance to solvents and chemicals.
There have been many attempts to improve the inadequate solvent resistance of powder coating compositions based on epoxy-functional polyacrylates and dicarboxylic acids or the mono- or polyanhydrides thereof. EP-A 0,299,420 thus describes diol- or polyol-modified polyanhydride crosslinking agents based on aliphatic dicarboxylic acids having 3 to 20 carbon atoms. Corresponding powder coating compositions with epoxy-functional polyacrylates as the binder yield lacquer films with good levelling properties, but which do not have the requisite resistance to solvents and chemicals at the low baking temperatures of approximately 140.degree. C. which are required in practice.
EP-A 0,509,392 and EP-A 0,509,393 describe powder coating compositions based on epoxy-functional copolymers and aliphatic or cycloaliphatic dicarboxylic acids, the anhydrides thereof or polyol-modified anhydrides. An essential feature of the invention in each case is the low baking temperature of 120.degree. C. which may be achieved. The low baking temperature is apparently attributable to the fact that the epoxy-functional copolymer contains 5 to 50 wt. % of tert.-butyl acrylate or tert.-butyl methacrylate in EP-A 0,509,392, and 35 to 50 wt. % of styrene in EP-A 0,509,393.
U.S. Pat. No. 4,346,144 describes powder coating compositions prepared from A) epoxy-functional copolymers, B) aliphatic dicarboxylic acids having 8 to 18 carbon atoms as the crosslinking-agent and C) an additional crosslinking component which may react with hydroxyl or carboxyl groups, such as alkylated melamine/formaldehyde resins, alkylated glycoluril resins, aliphatic glycidyl ethers and cycloaliphatic diepoxides. Blocked polyisocyanates are also mentioned as additional crosslinking substances in the specification and several examples.
For environmental reasons it is desirable to provide coatings which are completely free of emissions, e.g., solvents or the blocking agents released when the blocked polyisocyanates are heated to their deblocking temperature.
An object of the present invention is to provide a novel powder coating composition which does not suffer the disadvantages of the prior art, i.e:, which may be hardened at temperatures of below 160.degree. C. to yield a smooth, elastic coating that has improved gloss and is resistant to solvents and chemicals.
This object may be achieved with the powder coating composition according to the invention which is described in greater detail below. The novel powder coating composition is characterized by its specific composition and the specific molecular structure of the polyacrylate component A) used as the main component, which has both epoxide and cyclic carboxylic anhydride groups.
DE-OS 2,509,410 describes similar, but not identical, powder coating compositions. The differences reside in particular in the proportion of epoxy-functional monomers (maximum 25 wt. %), the composition of the non-functional monomers and the molecular weight distribution (M.sub.w /M.sub.n =1.6 to 3.0, preferably 1.7 to 2.2) of the copolymers which have epoxy and anhydride groups.